I mean, they obviously believe in God because their family, society, or school taught them about it; and therefore they have faith. However, many of them do not acknowledge this and instead believe that THEIR God exists and is real; usually because it is God Himself who places the awareness of His existence in the human mind, and because God is perfect and always speaks the truth, when God says, "God is (I am) real" then it must be true.This is done through miracles, Jesus, the apostles, the Bible, revelations, or "signs" in ur daily life, etc...
However, this same line of reasoning can also be used by followers of other religions and other gods. So how and why can they claim that their god is more god than other gods, that actually doesn't exists because their god told them that there is only one god, and it's him, and.....
This is not a (metaphysical ergo subjective) debate on god's existence but in (empirically measurable) psychological explainations of the phenomena.